Peru - Huarango Forest

The archaeobotanical record in the Ica Valley

Research at the Department of Archaeology of the University of Cambridge demonstrates how human interactions with the remarkable desert Huarango have influenced the historical ecology of the south coast of Peru:

16th Century Spanish conquistadores encountered one of the largest and most dazzling civilizations in history: the Inca Empire based in the Andes mountains. This empire was the culmination of some 5,000 years of sophisticated and unique cultural developments, whose roots lay along the arid Peruvian coast. The lush river oases that cross the barren south coast were once home to the Paracas and Nasca cultures, who flourished there from around 500 BC to 500 AD. These cultures are famous for their beautiful by textiles and ceramic artefacts, as well as their giant geoglyphs, known as the Nazca Lines.

These ancient people grew important domesticated plants such as maize, cotton, potatoes, tomatoes, pumpkins and chillies on the river floodplains. But, as in other arid American environments with intermittent water sources, wild plants always provided a crucial supplement to agricultural produce. The most important of these were huarango trees. Because of its deep roots, the tree reliably produces fruit regardless of erratic river flows. We find evidence of the importance of sweet, nutritious huarango pods to people's diets in the form of desiccated human excrement. Our excavations in the Samaca Basin of the lower Ica Valley reveal many of these, with the remains of pods eaten over 1,500 years ago.

The huarango grows slowly, and can live for over 1,000 years. The few
ancient trees left on the south coast were already old when the
Inca conquered the area 500 years ago. Its wood is harder than oak and
makes an excellent construction material and firewood, as local people
have long appreciated. Archaeological wooden artefacts of huarango still
survive almost perfectly preserved in this arid environment.

However the importance of Prosopis to ancient (and modern) humans goes far beyond its value as a resource. Prosopis is extraordinarily important to desert environments, providing islands of fertility and moisture. Today, the basins of the lower Ica Valley are almost completely abandoned, despite their extensive archaeological remains of human habitation, and ancient irrigation and agricultural systems. What could be the reason for this dramatic change?

Our excavations and surveys indicate several processes working together, some gradually and others suddenly, to cause eventual environmental change. One of these is the effect of sudden El Niño events. Archaeological strata dating to end of the Nasca Period (about 500 AD) are overlain by huge flood deposits that have buried some irrigation systems and left others high (and dry) as the river has cut down into the valley floor. However huge floods are only part of the archaeological story.

The old river terrace left exposed by the river's incision, and today without even a single blade of grass, turns out not only to be covered in the archaeological remains of its former populations, but also to contain the ancient remains of the many plants that once grew upon it. These have been preserved in the dry desert climate. They include pollen and desiccated plant macrofossils, including dozens of relict Prosopis tree trunks.

These data also offer a picture of gradual ecological change. The pollen
record shows that the basin was once densely forested with Prosopis trees. These were gradually cleared over time. Maize and cotton appear in the pollen record in association with Prosopis woodland. At some point towards the end of the Nasca Period and the beginning of the subsequent Middle Horizon, however, tree pollen practically disappears from this pollen record. This coincides with the occurrence of great El Niño floods. Removal of trees, especially ones with roots systems as deep as Prosopis,
exposed the river valley first to the effects of flood erosion, causing
river down-cutting; and then to the gradual effects of wind erosion.
The winds blowing in off the sea on the south coast are very strong and
over time have blown away the soil leaving the ancient canal courses
standing above the land surface today. Once the many beneficial effects
of Prosopis trees were removed, change became dramatic in this
fragile desert environment.

This story of anthropogenic change in ancient times is relevant to
sustainable use of these areas. Today the few remaining large trees
of the south coast are being rapidly converted to charcoal and the relict
forest lost to intensive agriculture. The consequences for the ecology
and also for the agriculture of the south coast can be seen in the abandoned
fields, erosion and floodplain desertification.

The research presented in publications has achieved considerable public impact, being reported in print and online in Nature, NewScientist, the bbc, Channel 4 news (uk), the New York Times, Los Angeles Times, El País and all uk broadsheet newspapers, among many others. For a sample of the coverage, please see:

Dr David Beresford-Jones is researching how human interactions with Prosopis have influenced the historical ecology of the Ica Valley, Peru. You can contact him at the George Pitt-Rivers Archaeobotanical Laboratory, University of Cambridge.